In the manufacture of semiconductor integrated circuits, a conductive metal, typically aluminum, is used to create the electrical interconnections between devices formed on the substrate. During fabrication, a layer of aluminum is sputtered onto the upper surface of the semiconductor sample. This process is carefully controlled to maintain uniformity and to produce the desired grain size in the aluminum layer. After the layer is formed, various etching steps are performed to define the proper pathways which define the electrical interconnections.
In the process of developing and thereafter monitoring metalization fabrication procedures, it is desirable to evaluate the size of the metal grains in the layer. Various contact techniques have been developed to determine the size of the grains. Unfortunately, these techniques tend to be slow and destructive so that the sample can not then be used to form a device. In addition these techniques cannot easily measure sub-micron size grains.
There has also been developed at least one noncontact technique which relies on the measurement of optical reflectivity of the sample to determine grain size. In this technique, an unfocused probe beam having a relatively large diameter compared to the size of the grains is directed to the surface of the sample. The power of the specularly reflected probe beam is then monitored. The latter apparatus can detect relatively small angle scattering effects. As the size of the grains in the layer decreases and therefore the number of grain boundaries increase per unit area, the amount of detectable small angle scattering effects are increased. The increase in the small angle scattering effects reduces the amount of power in the specularly reflected beam. Accordingly, information about grain size can be obtained by measuring the variations in the reflected beam power with a lower beam power indicating smaller grain sizes.
While the latter approach provides a noncontact technique it has certain drawbacks principally related to the use of the large diameter probe beam. Measurements using such a large beam will be influenced by field illumination effects. More significantly, the latter approach provides only indirect information and is not measuring actual grain size. In addition, the measurement will only yield an average grain size and will provide no information with respect to the distribution of the size of the grains within the layer.
Accordingly, it is an object of the subject invention to provide an apparatus for measuring the size of the grains in a metalized layer on a semiconductor sample.
It is a further object of the subject invention to provide an apparatus for measuring grain size which relies on optical reflectivity measurements.
It is another object of the subject invention to provide an apparatus which relies on optical reflectivity measurements to directly determine grain size.
It is still another object of the subject invention to provide an apparatus which utilizes a highly focused probe beam to derive information about grain size while avoiding adverse field illumination effects.
It is still a further object of the subject invention to provide an apparatus which can determine the distribution of the sizes of the grains in a metalized layer.